1. Field of the Invention
The present invention relates to polarization-maintaining optical fiber components which is useful in the optical fiber communication field, the field of sensors using optical fibers and the like, such as a polarization-maintaining optical fiber coupler which couples and branches lights while maintaining the polarization of light in optical fibers, and a polarization beam splitter which decouples and couples polarized waves perpendicular to each other, and a polarization-maintaining optical fiber to be used in producing those optical fiber components. This application claims the priority of Japanese Patent Application No. 11-234782, which is incorporated herein by reference.
2. Background Art
A polarization-maintaining optical fiber is designed by providing the stress distribution in a single-mode optical fiber with an anisotropical property to lift the degeneracy between two orthogonal modes that propagate in the optical fiber, thus yielding a difference in the propagation constant, so that coupling between modes is canceled. Accordingly, when light having a certain polarized wave enters an optical fiber, it propagates while maintaining only its polarized wave.
While there are various kinds of polarization-maintaining optical fibers, a well-known one is a stress applying type which has a stress applying section provided in the cladding. Depending on the shape of the stress applying section, this type is called a PANDA (Polarization maintaining AND Absorption reducing) fiber (hereinafter referred to as “PANDA fiber”), a bow tie polarization-maintaining optical fiber or an elliptic jacket polarization-maintaining optical fiber. Of those fibers, the PANDA fiber is widely used because of its large birefringent index and excellent polarization-maintaining characteristics.
FIG. 4 exemplifies a conventional PANDA fiber. This PANDA fiber 4 comprises a core 1 having a high refractive index, a cladding 2 provided concentrical to the core 1 and having a lower refractive index than that of the core 1, and two stress applying sections 3 arranged in the cladding 2 symmetrically to each other around the core 1 and having a circular cross section and generally having a lower refractive index than the cladding 2.
The stress applying section 3 uses a material of a relatively large coefficient of thermal expansion. In the process of manufacturing the PANDA fiber 4 by melt-drawing the preform of the optical fiber, therefore, different stresses are applied to the core 1 from the horizontal and vertical directions at the time glass is solidified. As a result, a large distortion is non-isotropically applied to the core 1, so that the PANDA fiber 4 will have a birefringent property.
Polarization-maintaining optical fiber components which comprise such polarization-maintaining optical fibers include a polarization-maintaining optical fiber coupler which branches and couples lights while maintaining the plane of polarization, a polarization beam splitter or a polarization beam combiner which decouples and couples polarized waves perpendicular to each other.
Those polarization-maintaining optical fiber components are manufactured by placing the cores of a plurality of polarization-maintaining optical fibers in close contact to one another and constructing an optical coupling section which causes optical coupling between optical fibers.
A fused elongation scheme and a polishing scheme can be used to make the cores of a plurality of polarization-maintaining optical fibers to closely contact one another. The fused elongation scheme however is advantageous from the viewpoint of reliability of workability.
The fused elongation scheme is a method of manufacturing a polarization-maintaining optical fiber component by laying a plurality of polarization-maintaining optical fibers side by side and heating, fusing and elongating their lengthwise portions in the lengthwise direction to thereby form an optical coupling section.
In the case of manufacturing a polarization-maintaining optical fiber component by the fused elongation scheme, it is necessary to observe the stress applying sections and align the polarization axes of a plurality of polarization-maintaining optical fibers in order to prevent crosstalk between perpendicular polarized waves.
A typical scheme for aligning the polarization axes is disclosed in, for example, Japanese Patent Application No. 02-271307. This scheme uses the fact that the refractive indexes of the cladding and the stress applying section differ from each other, and places a light source by the polarization-maintaining optical fibers, acquires the luminance distribution of fiber images by observing the polarization-maintaining optical fibers from the opposite side to the light source and determining the positions of the stress applying sections.
When the fused elongation scheme is used, the polarization-maintaining optical fibers are fused-elongated to be thin at the optical coupling section of the polarization-maintaining optical fiber component. Therefore, the light that propagates through the polarization-maintaining optical fiber leaks to the cladding portion from the core and enters a so-called air cladding state. Under this situation, because the single mode condition for the polarization-maintaining optical fiber is not satisfied, the bending of the polarization-maintaining optical fiber and optical coupling to a high-order mode occur due to an unevenness factor in the polarization-maintaining optical fiber. They eventually appear as an excess loss of the polarization-maintaining optical fiber component.
In the case of producing an optical component using an ordinary optical fiber, the refractive index in the optical fiber is uniform except for the difference in refractive index between the core and the cladding, and the core has a small diameter of several μm. Accordingly, the optical coupling section of the fused-elongated type optical component is elongated to be thinner so that the uneven refractive index in the fiber at the optical coupling section is not large enough to induce optical coupling to a high-order mode and does not raise a problem.
In the case of producing a polarization-maintaining optical fiber component using a polarization-maintaining optical fiber, by way of contrast, stress applying sections having a diameter of ten and several μm and a low refractive index are present in the cladding. The stress applying sections exit as the very large uneven refractive index portions in contrast with the case of using an ordinary fiber. As mentioned above, therefore, coupling of propagating light to a high-order mode occurs at the optical coupling section due to a difference in refractive index between the stress applying sections and the cladding. This increases the excess loss. This problem occurs prominently in the light along the fast axis where light leaks from the core occur frequently.
One way to solve this problem is the use of a polarization-maintaining optical fiber in which the refractive index of the stress applying sections is matched with the refractive index of the cladding, as disclosed in, for example, Japanese Examined Patent Application, Second Publication, No. 62-30602. As both refractive indexes are matched with each other in this method, however, it is difficult to observe the positions of the stress applying sections based on a difference in refractive index so that the aforementioned method of adjusting the polarization axis cannot be used.